This invention relates to gas turbine combustion systems and, specifically, to a fuel nozzle design which minimizes combustor damage during a combustion flame flashback or flame holding event.
A gas turbine combustor mixes large quantities of fuel and compressed air and burns the resulting mixture. Conventional combustors for industrial gas turbines typically include an annular array of cylindrical combustion “cans” in which air and fuel are mixed and combustion occurs. Compressed air from an axial compressor flows into the combustor. Fuel is injected through fuel nozzle assemblies that extend into each can. The mixture of fuel and air burns in a combustion chamber of each can. The combustion gases discharge from each can into a duct that leads to the turbine.
Combustion cans, designed for low emissions, include premix chambers and combustion chambers. Fuel nozzle assemblies in each combustion can inject fuel and air into the chambers of the can. A portion of the fuel from the nozzle assembly is discharged into the premix chamber of the can, where air is added to and premixed with the fuel. Premixing air and fuel in the premix chamber promotes rapid and efficient combustion in the combustion chamber of each can, and low emissions from the combustion. The mixture of air and fuel flows downstream from the premix chamber to the combustion chamber which supports combustion and under some conditions receives additional fuel discharged by the front of the fuel nozzle assembly. The additional fuel provides a means of stabilizing the flame for low power operation, and may be completely shut off at high power conditions.
A flashback or flame holding condition may occur in combustion cans having premix chambers. The premix chambers are not intended to support combustion. Flashback occurs when flame propagates into the premix chamber from the downstream combustion chamber, typically caused by momentary transient conditions. Flame holding occurs when a flame is initiated in the premixing zone, possibly by an external source such as a spark or hot foreign object ejected by the compressor, and the flame then stabilizes in a recirculation zone or weak boundary layer zone immediately downstream of the portion of the fuel nozzle assembly discharging fuel into the premix chamber. The damage resulting from flashback or flame holding may include burning combustor components not intended to be subjected to the heat of combustion. The damage caused by burning these combustor components may cause the components to malfunction and break up. If broken sections of the combustor flow into the combustion gas stream, they potentially may damage the hot gas path, e.g., turbine in the gas turbine.
Fuses in fuel nozzle assemblies prevent flame holding by diverting fuel away from the fuel nozzles for the premix chamber. The diversion of fuel from the premix chamber causes the abnormal flame to burn out and prevents further combustion in the premix chamber. However, conventional fuse designs, such as disclosed in U.S. Pat. No. 5,685,139, are not suited to all types of fuel nozzle assemblies. Accordingly, there is a need for novel designs of fuses.